As known, civil transport airplanes should be pressurized, as upon a cruise flight, an airplane flies at an altitude being often higher than 30,000 feet (about 9,000 meters), for which the external air is too low in oxygen (and also too cold and too dry) for being compatible with life. Thus, pressurizing systems are provided in airplanes for keeping on board a breathable atmosphere. In particular, the international aeronautic regulation states that any public transport airplane flying at an altitude higher than 20,000 feet (about 6,000 meters) should be pressurized and that it should establish in the cabin an equivalent altitude which does not exceed 8,000 feet (about 2,400 meters) upon a normal flight.
It may however occur that, as a result of a breakdown or a failure, the pressurization of the airplane could no longer be maintained at an acceptable level. A regulatory procedure then compels the pilot to have the airplane descent, as quickly as possible, at a breathable altitude of 10,000 feet (about 3,000 meters) or at the current security altitude if it is not possible to descent as low as 10,000 feet because of the relief. Such a procedure is referred to as an emergency descent.
In such a case, the crew is responsible for different tasks related to initiating the descent, as well as the adjustment of parameters of the descent (speed, target altitude, lateral trajectory, etc.) and this until the airplane flies level at low altitude.
It may happen, however, although very rarely, that the crew is no longer able to apply the above described procedure, for instance in the case of a pressurization breakdown as a result of which the crew has lost consciousness.
The airplane is, in such a case, unattended, while it is absolutely necessary to carry out an emergency descent. If, in such a situation, the autopilot is activated, the flight is continued automatically until the kerosene supplies are totally exhausted.
In order to avoid such a situation, an autopilot system is known, allowing, when it is triggered, to carry out the emergency descent automatically, that is without requiring the help of a pilot. Moreover, triggering such an automatic emergency descent could be carried out either manually by the pilot, or also automatically.
In particular, from document FR-2,928,465, a specific method is known for automatically controlling an emergency descent of an aircraft. According to this method, when an emergency descent automatic function is triggered, the following successive operations are carried out:
a) a set of vertical setpoints is automatically determined comprising:
a target altitude representing an altitude to be reached by the aircraft at the end of the emergency descent; and
a target speed representing a speed that the aircraft should respect upon the emergency descent;
b) a set of lateral setpoints is automatically determined, representing a lateral maneuver to be carried out upon the emergency descent; and
c) the aircraft is automatically guided so that it simultaneously respects said set of vertical setpoints and said set of lateral setpoints until reaching said target altitude that it subsequently maintains, said automatic guidance being able to be interrupted by an action of the pilot of the aircraft.
Furthermore, this known method provides particular devices that automatically trigger the emergency descent function, taking into account the variation of altitude of the cabin, that is the variation of pressure inside the cabin.
As far as the determination of a target altitude is concerned within the context of an automated emergency descent, the following is known:
from document U.S. Pat. No. 4,314,341, an automated emergency descent to a security altitude, the value thereof being inclusively fixed to 2000 feet (about 3600 m). Such a value corresponds to a physiologically breathable and satisfactory altitude but it could be lower at the highest grounds (Alpes, Himalayas, Andes, Rocky Mountains, etc.). Therefore, it is not satisfactory to ensure a secured end of maneuver, should a crew be unconscious (as a result of possible collision with the ground);
from document U.S. Pat. No. 6,507,776 B1, a coupling between an autopilot and a GPS system having a data base wherein values of altitude are stored for all reliefs, having the altitude higher than or equal to a fixed maximum value. Such a GPS system is provided with a device for identifying the relief along the current trajectory. Such a device allows the autopilot to be provided with the lowest possible security target altitude, being available adjusting the heading of the aircraft if needed, for bypassing the ground. Such a device has the drawback of potentially directing the aircraft outside the area covered by the initially followed air traffic way. The associated risk involves increasing the workload of the crew when they regain conciousness, as the aircraft is likely to fly far from the initially followed flight itinerary, and, moreover, may not have enough kerosene available for reaching the closest deviating airport. Additionally, such a margin of only 1,000 feet with respect to the ground, may not be satisfactory for covering all the possible fluctuations of barometric pressure along the emergency descent; and
from document U.S. 2007/0043482, another device integrated into an autopilot able to carry out automatically an emergency descent to a security altitude, the calculation thereof being based on security minimum altitudes of the Minimum Safe Altitude (“MSA”) type. More precisely, a data base containing the MSA altitudes is used for determining the associated security altitude, either at the current flight itinerary, or, should it exist, at a deviation trajectory provided by the airline company. When the airplane is outside the flight itinerary or outside a deviation way, the security altitude is calculated from the data base of the ground, taking as a value, the maximum altitude on a trajectory maintaining the current heading, to which there is added a security margin of 1,000 feet or 2,000 feet (in the case of a hilly area). However, this security margin with respect to the ground could decrease significantly if no update of the target altitude has been carried out for taking into account the barometric pressure reference.
As known, the local atmospheric pressure is subject to non negligible variations on a distance such as the distance covered upon an emergency descent, for instance approximately 40 NM.
Moreover, the values of the security altitudes MSA or Minimum Off Route Altitude (“MORA”) issued from known data bases available by the Flight Management System (“FMS”) are barometric altitude values, referenced with respect to the level of the Mean Sea Level (“MSL”).
Moreover, upon a cruise flight, the barometric reference of the on-board instruments is generally a standard reference (STD), corresponding to a nominal pressure of 1013.25 hPa. This reference is used by all the airplanes in a cruise phase as well as by the air traffic control, and allows for some consistency between the exchanged altitude information. Flight levels are taken into account. When an airplane flies at a flight level FL350 for instance, this means it is flying at an altitude of 35,000 feet, referenced at 1013.25 hPa/15° C. (standard ISA model). As the local atmospheric pressure constantly progresses when the airplane crosses different masses of air, the airplane actually changes elevation with respect to the sea level following the same flight level. As the reference remains the same for all the air traffic, this does involve any problem for the air control, for knowing precisely the relative altitudes of each of the airplanes and ensuring a satisfactory security level.
Taking into account all those constraints, it is understood that if the barometric reference of the airplane is the standard reference STD and the MSL referenced security altitude is known, it is necessary to determine the local QNH pressure (e.g., “QNH” is the Q-code defined as “barometric pressure adjusted to sea level”) of the flown over point, i.e. the pressure converted to the level of the sea, for precisely updating the target altitude on the local reference while keeping the barometric control of the on-board instruments based on the standard reference (STD). This last point is important, as it is desired not to modify the barometric settings of the cockpit for two main reasons:
keeping a consistency with the usually used barometric reference in a cruise, by all the traffic and by the air traffic control; and
allowing the pilot to quickly find his marks in the case where he would have lost conciousness and subsequently regain conciousness as a result of a depressurization at the origin of the automatic emergency descent.
It is known that the pressure QNH is provided by ground stations being located in the vicinity of airports, but there is no simple means allowing the local QNH pressure to be obtained automatically.
Thus, in the absence of an update of the target altitude for taking into account the differences of pressure reference, the security margins are likely to be considerably reduced with respect to the ground.
The present invention aims at solving the above mentioned drawbacks. It relates to a method for updating a target altitude intended for an emergency descent of an aircraft, said target altitude representing the altitude to be reached by the aircraft at the end of the emergency descent.